I actually had your problem. I got the best result by moving her to a separate tank. I have a 5 gallon set up with a small box corner bubble filter, no gravel or lighting, java moss and floating water sprite.
I use the same exact tank water as the main tank with the only difference is water temp. After she was square I nabbed her and plopped her in the five. She gave birth about a day and a half later during early morning hours.
She had about 20 plus fry and I love that she isn't a fry eater!
By herself she isn't stressed by other fish, especially the male platy lol. Also the fry survival rate is higher than in your main tank.